An antiskid brake that prevents the wheels of a vehicle from locking due to braking generally detects a slip factor of the wheels, and controls the braking pressure so that the slip factor is within a predetermined range. This predetermined range is generally set to be centered on the slip factor at which the braking force is maximum.
When the slip factor exceeds this predetermined range the braking force is decreased, and when it returns to the predetermined range, the brake pressure is increased.
In order to stabilize the slip factor as rapidly as possible, Tokkai Hei 2-3564 published by the Japanese Patent Office in 1990 proposes an algorithm wherein proportional integral control is applied to the pressure increase process. According to this algorithm, a total pressure decrease amount during the immediately preceding pressure decrease operation is stored, and when it is increased, an amount obtained by multiplying the total decrease amount by a fixed proportion is increased in one operation as a proportional pressure increase. Subsequently, the pressure is increased in small steps by integrating pressure increase amounts of smaller value until the predetermined range is exceeded. In this way, the time required for the slip factor to stabilize on the same road surface is reduced.
However, this control is effective only when road conditions are constant. For example, when the vehicle moves from a road with a high frictional coefficient such as a dry, surfaced road onto a road with a low frictional coefficient such as a wet road or icy road, the slip factor increases sharply, so the braking pressure is largely reduced until the slip factor returns to within the predetermined range.
When the slip factor returns to the predetermined range, the braking pressure is increased, and according to the aforesaid algorithm, the proportional pressure increase is determined based on the immediately preceding total pressure reduction. However, the immediately preceding pressure decrease is large due to the change of frictional coefficient, and consequently the proportional pressure increase amount based on this value may be excessive for the low frictional coefficient so that the slip factor immediately leaves the predetermined range. In this case, the slip factor oscillates with a large amplitude, and there is an adverse effect on the stability of the slip factor.